In recent years, much activity and invention in the art of polyesters has centered around the development of articles made from poly(ethylene terephthalate) (PET), and to a lesser extent, poly(butylene terephthalate). Containers constructed of PET have recently found wide acceptance in the packaging of food stuffs, medicines, and consumer products.
Despite the popularity of PET in container manufacture, there are several drawbacks to the use of PET. Although PET is considered a high barrier polymer, the use of PET containers for beer has heretofore been avoided due to the rapidity with which beer loses its flavor, due largely to oxygen migration into the bottle. Further, because of the high processing temperatures involved in the manufacture of PET bottles, a significant amount of acetaldehyde by-product is formed during extrusion and molding. This compound, at relatively low concentrations, readily imparts an undesirable or bad after taste to many food stuffs packaged in PET.
U.S. Pat. Nos. 4,049,631; 4,098,769; and 4,110,315 all to Santos Go, relate to copolyesters containing sulfone units therein and having a glass transition temperature of 75.degree. C. or higher. Unoriented poly(ethylene terephthalate) generally has a glass transition temperature of about 72.degree. C. Although the copolyesters of Go tend to have lower oxygen and carbon dioxide permeabilities than pure poly(ethylene terephthalate), they also give higher glass transition temperatures, ostensibly for the purpose of producing materials which can be subjected to higher temperatures than that of pure PET.
U.S. Pat. No. 4,145,517 also to Santos Go, relates to copolymers of polyesters also containing sulfone units therein. The product of this patent is used for developing various items such as packaging for food stuff, molded containers for carbonated beverages, medicine bottles, and the like. Due to the use of the particular type of sulfone, the copolymer has a very high glass transition temperature, that is at least 121.degree. C. Accordingly, the copolyester exhibits improved properties as well as good heat stability. Additionally, there is somewhat of an improvement with regard to oxygen and carbon dioxide permeability resistance.
The Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition, 1978, Volume 3, at page 480, states that with regard to factors affecting barrier properties, a truly good barrier polymer must have some degree of polarity such as contributed by nitrile, etc. groups, high chain stiffness, inertness, close chain-to-chain packing, order, crystallinity or orientation, some bonding or attraction between the chains, and high glass transition temperatures. At page 487, it is noted that the permeation of carbon dioxide is an important factor in defining a high barrier polymer, especially for use with carbonated beverages since carbon dioxide permeation rates are three to four times that of oxygen in almost all polymers.
At page 489, it is stated that major use of high barrier polymers is packaging, especially food and beverages, as replacement for glass or metal containers. It is then stated at the bottom of page 489 as well as on page 490 that many foods are very sensitive to oxidation which cause flavor changes or discoloration. Moreover, it is noted that loss of 10 percent or more of carbonation can be easily detected by taste and that beer flavor is affected by oxygen levels of less than 2 parts per million.
U.S. Pat. No. 2,965,613 to Milone relates to copolymers of ethylene terephthalate and ethylene isophthalate and is not pertinent because it does not relate to high barrier packaging materials with respect to oxygen or carbon dioxide, or to lower acetaldehyde production.
U.S. Pat. No. 4,234,708 relates to extrusion grade polyethylene iso/terephthalate copolymers having less than 15 percent by weight of isophthalic acid and hence does not have low glass transition temperatures, or the like.
U.S. Pat. No. 3,733,309 relates to biaxially molded poly(ethylene terephthalate) with regard to making containers.
From the above prior art patents and article, it can be seen that there is a relationship in polymers between the permeability resistance and higher glass transition temperatures. Accordingly, it was totally unexpected that the use of a polyester compound having a low glass transition temperature such as certain polyisophthalates, e.g., poly(ethylene isophthalate), and copolymers thereof would have large increases in resistance to oxygen and carbon dioxide penetration. Moreover, the polyisophthalates or copolymers of the present invention process at lower temperatures and thus generate reduced amounts of various undesirable by-products such as acetaldehyde which imparts undesirable taste to various items such as carbonated beverages contained by the polymer.